Keep track of AST stack in AST instance, rather than as a static variable.
Keep track of AST stack in AST instance, rather than as a static variable.

I ran into issues when enabling debug mode, that unclosed nodes from one
test (sometimes on purpose), caused unclosed nodes errors on other tests.
By tracking the AST stack on `this` rather than on the `AST` class directly,
this can no longer happen, and each test runs in isolation.

glayzzle/php-parser,glayzzle/php-parser